The South American Leaf Blight (SALB), caused by the fungus Microcyclus ulei, is the major rubber tree disease in all Central and South America. A population of 192 progeny individuals derived from a cross between a resistant clone and a susceptible cultivated clone was planted in a field trial in French Guiana in order to evaluate the resistance parameters under real infestation conditions. The resistance type (RT), the presence of stromata (ST) and the level of attack (AT) were observed 20-times on a 22-months period, and semi-quantitative evaluation of stromata was registered only once. The search for QTLs was performed using the Kruskal-Wallis test, Interval Mapping and the Composite Interval Mapping method. One major QTL located on linkage group g13 was detected on the RO 38 map, responsible for 36 to 89% of the phenotypic variance of resistance. This resistance QTL corresponds to one that had previously been detected under controlled conditions of infestation and we called it M13-1bn. Surprisingly, the effect of this QTL was larger under natural conditions of infestation than under controlled inoculation. Other minor QTLs (four on the RO38 map and one on the PB 260 map) were also detected. The type of resistance brought by M13-1bn, as well as its durability, are discussed. Applications for rubber tree breeding programs are considered.